1. Field of the Invention
The present invention relates to a liquid jetting device that jets, for instance, a plurality of fine colored-liquid droplets from a plurality of nozzles to form dots on a print medium for drawing predetermined characters and images.
2. Description of the Related Art
One example of such a liquid jetting device, the inkjet printer, has with the popularization of personal computers digital cameras and the like become popular not only in offices but among private users as a result of its ability to produce high quality color prints of a high quality at low cost.
Such inkjet printers are generally constructed to produce the desired printed matter by drawing predetermined characters and images on the print medium through jet of liquid ink droplets from print head nozzles and formation of fine ink dots on the print medium as a movable body, which is integrally equipped with ink cartridges and print heads and known as a carriage, moves back and forth over the print medium in the direction which intersects with the conveyed direction of the print medium. By including of ink cartridges of four colors including black (and yellow, magenta, and cyan) and a print head corresponding to each color, the inkjet printers are configured to allow both monochrome and, through combination of the colors, full color printing (inkjet printers further including arrangements for additional colors, such as light cyan and light magenta, to give six, seven or eight colors are also in use).
The inkjet printers of the type which execute printing by passing the inkjet head on the carriage back and forth in the intersecting direction with the conveyed direction of the print medium must make from around ten passes to several tens of passes to make a good print of an entire. Contrastingly, in inkjet printers of the type which avoid use of a carriage by providing long inkjet heads (which need not be integrated with the cartridges) of substantially the same width as the print medium, there is no need to move inkjet head in the width direction of the print medium and the printing can be completed in a single pass. The inkjet printers using the former method are generally called “multipass (serial) inkjet printers”, and inkjet printers using the latter type are generally called “line-head inkjet printers”.
At present, further improvements in “gradation” are being demanded in these types of inkjet printer. Gradation refers to states of concentration of each color in a so-called pixel expressed as an ink dot. A size of the ink dot, which dependent on the concentrations of the colors in the pixel, is called a degree of gradation, and a number for the degree of gradation possible using ink dots is called a gradation number. High gradation is used to mean a large gradation number. To change the degree of gradation, it is for instance necessary to change a driving pulse to an actuator provided in the inkjet head. If the actuator is a piezoelectric device, an amount of displacement (deformation) of the piezoelectric device (more accurately, a diaphragm) will increase as the applied voltage to the piezoelectric device is increased, and it is therefore possible to change the degree of gradation.
In JP-A-10-81013, for instance, a driving signal is generated by combining and linking a plurality of driving pulses having differing peak values. The generated driving signal is outputted to all piezoelectric devices corresponding to nozzles of a given same color provided on the inkjet head. The desired degree of gradation in the ink dot is then achieved by selecting, for each nozzle, a driving pulse corresponding to the degree of gradation in the ink dot to be formed and supplying the selected driving pulses to the corresponding piezoelectric devices to jet ink droplets.
A method for generating the driving signals (or driving pulses) is recorded, for instance, in FIG. 2 of JP-A-2004-306434 which is described below. In this method, the inkjet printer reads data from a memory storing the driving signal data, converts the read data to analog data using a D/A converter, and supplies the resulting driving signal to the inkjet heads via a current amplifier. Since the piezoelectric device is a charge-discharge actuator, the driving signal causes the charge-discharge actuator to charge and discharge. The current amplifier circuit is, as shown in FIG. 3 of the same application, constructed using a charge-use transistor and a discharge-use transistor connected in a push-pull configuration. Further, the current amplifier configuration makes use of a high power source potential, and amplifies the driving signal using so-called linear drive. However, in current amplifiers with this type of configuration, a potential difference between the power source potential and the driving signal for charging the charge-discharge actuator and a potential difference between the ground potential and the driving signal for discharging the actuator are both large, and such amplifiers therefore consume a great deal of power. The majority of the consumed power is dissipated as heat, and large transistors and heat sinks are therefore required. The large size of the heat sinks, in particular, is a serious hindrance with regards to layout.
To overcome this disadvantage, in the inkjet printer disclosed in JP-A-2006-272907 a source potential adjusting transistor is provided between a power source and the charge-use transistor, and a charging source potential adjusted using the source potential adjusting transistor is supplied to a collector of the charge-use transistor via a ripple filter. Further an earth connection (discharging connection) potential adjusting transistor is provided between an earth connection (discharging connection) and the discharge-use transistor, and an earth connection (discharging connection) potential adjusted by the earth connection (discharging connection) potential adjusting transistor is supplied to a collector of the discharge-use transistor via a ripple filter. Such an arrangement reduces the potential difference between the charging source potential and the driving signal for charging the actuator and the potential difference between the earth connection (discharging connection) potential and the driving signal from the discharging actuator, thereby reducing power consumption.
However, in the inkjet printer disclosed in JP-A-2006-272907, since only a single charging source potential adjusting transistor is provided between the power source and the charge-use transistor and only a single earth connection (discharging connection) potential adjusting transistor is provided between the earth connection (discharging connection) and the discharge-use transistor, it is not possible to sufficiently close the gap between a charging source potential preliminary adjusted signal supplied to the charge-use transistor and the charging potential of the charge-discharge actuator, or to close the gap between an earth connection (discharging connection) potential preliminary adjusted signal and the discharge potential. Since, the potential differences represented by the gaps cannot be reduced, the disclosed arrangement is incapable of sufficiently reducing power consumption.
The present invention provides a liquid jetting device capable of reducing both the potential difference between the charging source potential preliminary adjusted signal and the driving signal for charging the charge-discharge actuator and the potential difference between the discharging connection potential preliminary adjusted signal and the driving signal from the charge-discharge actuator when discharging, and thereby allows a reduction in power consumption.